The present invention relates to a motor using Hall effect elements, and more particularly to a brushless motor housing a multi-pole magnet as a rotor and Hall effect elements for detecting positions of magnetic poles of the rotor magnet to switch polarities of currents supplied to stator coils.
In a disk record player for playing back a disk record and magnetic tape recorder and player for recording and reproducing a signal on a magnetic tape, it is desirable to use a brushless motor in order to minimize vibration and noise. The brushless motor is particularly advisable for the motor used in the disk record player in which a rotary shaft of the motor is directly coupled to a rotary shaft of a turn table to directly drive the turn table, because the vibration of the motor is directly transmitted to the turn table. A motor which uses a multi-pole magnet as a rotor and in which the positions of magnetic poles are detected by Hall effect elements to switch the polarities of currents supplied to drive coils has been known. In one example of this type of motor, a disk-shaped multi-pole magnet is attached to a rotary shaft of the motor, which disk-shaped magnet is circumferentially divided into a plurality of magnetic poles each magnetized in the direction of thickness thereof in an opposite polarity to its adjacent magnetic poles. A pair of drive coils are disposed to face the disk-shaped magnet. Each of the pair of drive coils is wound in star-shape and has drive portions which extend outward and generally radially from the center of a rotating axis. The pair of coils are angularly displaced from each other by an electrical angle of 90.degree.. A pair of Hall effect elements are disposed within a magnetic field of the disk-shaped magnet and angularly displaced from each other by an electrical angle of 90.degree.. The motor has an axially extending magnetic air gap, and the pair of Hall effect elements detect the polarities of magnet poles of the disk-shaped magnet located on the respective Hall effect elements, and these output signals from the respective Hall effect elements are supplied to the pair of drive coils to generate driving magnetic fluxes therein. More particularly, one of the pair of Hall effect elements is disposed near one of the pair of drive coils so that said one Hall effect element detects the polarity of the magnetic flux which links to said one drive coil and the output signal from said one Hall effect element is supplied to said one drive coil, while the other Hall effect element is disposed near the other drive coil so that said other Hall effect element detects the polarity of the magnetic flux which links to said other drive coil and the output signal from said other Hall effect element is supplied to said other drive coil, whereby driving magnetic fluxes are generated in the respective drive coils.
In this motor, the polarities of the magnetic fluxes which link to the pair of drive coils are detected by the pair of Hall effect elements and the output signals therefrom are amplified and then supplied to the respective drive coils. Accordingly, the characteristics of the pair of Hall effect elements must be identical to each other. If the characteristics of the pair of Hall effect elements are unbalanced resulting in a difference between the output signals, there occurs a difference between driving torques generated in the pair of drive coils resulting in non-uniform rotation of the motor and vibration.
It is, however, difficult to obtain a pair of Hall effect elements of balanced characteristic. In the past, therefor, the currents supplied to the respective Hall effect elements have been controlled or the gains of the amplifiers for amplifying the output signals of the Hall effect elements have been adjusted to equalize the magnitudes of the driving torques generated in the pair of drive coils. The Hall effect element has a pair of current supply terminals and a pair of output voltage terminals. One of the pair of current supplying terminals is a common electrode to the pair of output voltage terminals and output voltages are developed between one of the pair of output voltage terminals and the common electrode and between the other output voltage terminal and the common electrode, respectively. A differential voltage between those two output voltages is referred to as an unbalance voltage. The unbalance voltage changes in proportion to the currents supplied to the pair of current supply terminals. Accordingly, it is not desirable to control the currents supplied to the Hall effect elements to adjust the driving torques because the unbalance voltage of each of the Hall effect elements is changed thereby. When the gains of the amplifiers are adjusted, gain control circuits must be included in the amplifiers. This leads to the increase of the cost of the amplifiers. In addition, it is difficult to establish optimum gain.